Professor Bill Manning proved to be an enthusiastic and knowledgable guide

When the emperor Septimius Severus paraded into Leptis Magna in 203AD the locals erected a ceremonial arch. They dressed it in marble and bronze and slapped on a toadying frieze to flatter him and his family.

When groups turn up at Leptis today their tour buses have to do a U-turn across a dual carriageway and then, reversing on to the highway, take two bites at getting through a narrow gate. What a difference 1,800 years make.

However unimposing the entrance, Leptis is one of the supreme spectacles of the classical world. But Leptis is in Libya and modern Libya doesn't go in for the imposing, even at one of its five World Heritage sites. In fact, if Colonel Gaddafi turned up with Leptis Magna at an Antiques Road Show you can't help wondering whether he wouldn't be one of those whose jaws drop when they hear just how much their item is worth.

It is only in the past four years, since sanctions were lifted, that the country has fully re-entered the international fold. In a land sequestered for as long as Libya was, everything is unfamiliar. At Tripoli airport a sign read: "Partners, not wage workers," which made it sound like John Lewis.

But then there was the first of countless photographs of The Leader, Col Gaddafi, in one of a dozen permutations of desert robes, army uniform and sunglasses. The Leader's collection of shades must match Imelda Marcos's assortment of shoes.

Before I was allowed to leave the airport three "partners" had to shuffle through my passport and two machines X-rayed my luggage. It took a third X-ray to get into the hotel. No, these days Libya doesn't do imposing. But it did once. The country has some of the largest and most magnificent remains of the ancient Mediterranean.

I was with a group of archaeology buffs, folk who dig digs, if not literally, who get a kick from Corinthian capitals. There were 19 of us, travelling with a holiday company that does nothing but arrange trips like this to the ancient world. Our tour, starting and ending in Tripoli, took us the breadth of Libya.

Our average age was 65 and the oldest among us was 82. There were business people, academics, lawyers and civil servants. One had been a judge for the Booker Prize. Some had studied ancient history at university; others discovered their fascination for archaeology on childhood holidays.

Between them they had visited most of the principal sites around the northern shores of the Mediterranean. Now they had come to Africa.

It was an erudite bunch whose erudition was about to be extended by a week in the company of our own lecturer. Three sets of people made this trip, the imperial Romans, the ancient Greeks and the Emeritus Professor of Archaeology at Cardiff University, Bill Manning. An expert on the Roman Empire, he spoke with the freshness and familiarity of someone who had only just got back.

He was both guide and guru. As we picked our way through the stumps of brick that had supported the heated floor of a Roman bathhouse, he cautioned, "Don't trip over the hypocaust pilae!" – not the sort of instruction you get on your average coach tour. Nor would he allow his enthusiasms, or our dawdling, to delay the schedule. "You have to keep going or you will end up as one of the sights of Libya yourself," he called to a straggler at Cyrene, where his voice, which has boomed through a hundred lecture rooms, now reverberated off the stones of the agora.

He would never let the ancients get above themselves. In Tripoli's fine National Museum we stood in front of a statue of a headless Roman ready to offer our admiration. Bill intervened: "It is the indoor variety of the garden gnome, produced by the 10,000. Much more important is the mosaic behind it." And he pointed out the figure of a man standing on a small trolley, being wheeled into an arena to be devoured by lions. "Not sure I would want it on my dining-room floor," he added.

Bill's enthusiasms were infectious. One was the harbour at Leptis. "I can show you any number of theatres and basilica, but I can't show you another Roman harbour. It's unique," he said as he led us out along a mole, the sea to our right. The dock it once protected silted up centuries ago, but the massive stone blocks forming the quay are still there, level and immaculately aligned, and upon them stand the foundations of warehouses and the base of a lighthouse.

From here were shipped slaves, ivory, gems, wild animals and olive oil. This was tough, industrial architecture with none of the showy finesse of the city. The stone rings, where the ships tied up, are as big as skips. "This is one of the sites that excite me, I confess," said Bill wistfully when it was time to leave.

The port was a gift of Septimius Severus. Born in Leptis in 145AD, he was the local boy who made so good he became emperor, the only African who did. (He died, incidentally, in York.) Leptis, already rich from olive oil, profited mightily from his patronage, a relationship celebrated in the triumphal arch.

The arch survives, virtually intact; its ingratiating marble frieze is in the National Museum. Like other monumental buildings it owes its condition first to the desert and much later to the Italians. Destroyed by earthquakes, Leptis was buried by encroaching sand and then in the 1920s, excavated and meticulously restored by the Italians during their colonisation of Libya.

The restoration was not entirely altruistic. Mussolini, Italy's Fascist dictator, was more than happy for the world to be reminded of his country's old penchant for imperialism. In 1936 Leptis provided him with a suitably grandiose dais from which to make a speech: you can still see the fence erected to stop him falling off the top of the nymphaeum. "Archaeology as politics," sniffed Prof Manning.

Whatever the motive, the restoration left Leptis, and other Libyan sites such as Sabratha, as much more than mere ruins. This is the spoor of real people in the real places where they lived; where they traded, gossiped, politicked, went to the theatre and the lavatory – there are great examples of both – watched chariots race and gladiators fight, where they whinged, giggled and grieved. Imagination races down paved and colonnaded streets, through amphitheatres, baths and basilicas, headily rummaging among their statues, inscriptions and mosaics for momentary citizenship of ancient Rome.

In Leptis we were lucky. Past the great 8,000-seat theatre, outside the city gates, we came to the coast and a low ridge of sand dunes. There, huddled at the back of the beach, was what looked like a couple of concrete Nissen huts with two igloo-shaped shelters wedged between them.

Bill pushed at an unlocked metal door and we entered the exclusive realms of a gentlemen's bathhouse, in its day a sort of Boodle's with steam rooms. The Leptis Hunting Baths' day was the late 2nd or early 3rd century. They get their name from vivid frescoes of wild animals – leopards and lions – being stuck with spears by curly-haired chaps on safari wearing what appear to be stab jackets and Y-fronts. "These are the only baths in the Roman world preserved pretty much as they were and extremely important in the history of Roman architecture," Bill enthused. "The Pantheon [in Rome] is the only other place you can stand with a Roman dome overhead."

Leptis and Sabratha are in Tripolitania. The second half of the trip took us east to Cyrenaica, the only part of North Africa to be settled by the Greeks. At Tocra, a "mongrel" site, according to Bill, the Byzantines built Christian churches from Roman masonry engraved in Greek.

We had flown to Benghazi, a city that seemed brighter and livelier than sombre Tripoli. My hotel room had not been swept and there were no hangers in the wardrobe; others were without towels or soap. Tourism in Libya is still a bit of a novelty and not without its complications. Besides a visa, you need to have your passport information translated into Arabic. Individual travel is not encouraged: it is much simpler to go in a group. We were accompanied by a delightful minder, a "tourist policeman" whose main, valuable, role seemed to be to bag us the best parking places.

Credit cards are all but useless, but mobile phones work; food is fresh, if repetitive. In the understatement of our tour operator: "Generally, meals are not the high point of the tour." There is an absolute ban on alcohol. Offset against all of that is the charm and humour of the Libyans. One senses they recognise that life in the Great Socialist People's Libyan Arab Jamahiriya demands a sense of the ridiculous.

Our days were intense. We would be on the road by 8am or 9am, have a lecture from Bill in the coach, spend anything up to four hours at each site and some days not be back until 6.30pm. There were few people around; we had some ruins to ourselves. Nor was there much management, no ropes or barriers to stop you clambering where you wanted. With more visitors such freedom would be seen as neglect.

We were in the Jebel al-Akhdar – the Green Mountains. They are stony hills spattered with dark green scrub, pine and olive trees. On our final day we picnicked in Cyrene, the biggest and most important of the Greek cities. We sat in the sunshine at the Temple of Zeus, a temple larger than the Parthenon, munching sandwiches beneath burly columns raised in the 6th century BC to the king of the Gods. This was Libya at its most imposing.

Libya essentials

Peter Hughes travelled with Andante Travels (01722 713800, www.andantetravels.co.uk), which organises similar archaeological tours in other parts of the world, including Europe and South America.

The next trip to Tripolitania and Cyrenaica in Libya, with a different lecturer, is in mid-October. The price, £1,925 for eight nights, includes all flights, meals and site entry fees. Availability is limited, but there will be two tours in spring and autumn next year.